In recent years, the demand for the capacity increase and size reduction of the secondary batteries has been growing with the spread of portable personal computers and portable telephones. To this end, the development of lithium ion secondary batteries having a high capacity has been widely made.
As positive electrode materials for a lithium ion secondary battery there have been widely used LiCoO.sub.2, LiCo.sub.1-x Ni.sub.x O.sub.2, LiNiO.sub.2, and LiMn.sub.2 O.sub.4 which are positive electrode materials for the secondary battery having a high potential. On the other hand, as negative electrode materials there have been normally used carbonaceous materials. Such the carbonaceous material acts as an electrode active material which reversibly intercalate and deintercalate lithium ion during charging and discharging and thus constitutes a so-called rocking chair type secondary battery electrochemically connected with a nonaqueous electrolytic solution or solid electrolyte.
Examples of carbonaceous materials to be commonly used as negative electrode materials include graphite-based carbon material, pitch coke, fibrous carbon, and high capacity type soft carbon calcined at a low temperature. However, carbonaceous materials are disadvantageous in that they have a low specific gravity of 2.26 (graphite). Therefore, if they are used in an amount such that lithium intercalating capacity reaches the stoichiometric limit (372 mAh/g), it is difficult to design the battery capacity as high as desirable. As negative electrode active materials having a high capacity density surpassing the carbonaceous materials there have been disclosed negative electrode active materials made of a composite oxide mainly composed of tin oxide in JP-A-6-60867, JP-A-7-220721, JP-A-7-122274, and JP-A-7-288123 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Processes for preparing these negative electrode active materials are also disclosed in the above cited patents.
These composite oxide negative electrode materials are synthesized by calcining at a high temperature of 1,000.degree. C. or more.
However, the foregoing synthesis process has a handling problem in production due to corrosion on the synthesis container. Further, the contamination by impurities eluted from the container causes variation of the physical properties of the electrodes. The material prepared by a melting method is ground to adjust its particle diameter before use as an active material for the battery. However, the material thus ground is liable to wide distribution of particle diameter that can be-a factor for instabilizing the battery performance. Further, since the group of particles having such the wide diameter distribution has a relatively small surface area, the effective surface area taking place in the intercalating reaction of lithium ion decreases, reducing the high current charge-discharge (high rate) efficiency and capacity of the battery. Moreover, in the melting method, various starting material powders, i.e., metal oxide powders are each melted, and then mixed in the form of dispersion. Therefore, it is difficult to uniform the metal elements on a molecular basis, making it difficult to uniformilize the composition distribution in the particle. There have been an apprehension that such a uniformity problem can affect the stability of charge-discharge characteristics of negative electrode materials.